Metal-lined through-holes and metal wrap-around edges provide an electrical connection between first and second sides (front and back) of a substrate at selected locations. The metal-lined through-holes can also function as heat sinks. One technique for making the hole or via, to which metal is subsequently applied to make the metal-lining or into which metal is applied to provide a filled via, is laser drilling.
When a hole is laser drilled through a substrate such as a ceramic plate, the wall of the resulting hole often includes a thin zone of material that is crack-networked and somewhat crumbly or friable, and which has some glassy character. This wall material is frequently referred to as slag or melt. Not infrequently, the front and back sides of the substrate proximate the laser drilled hole include small cracks.
It should be noted that the existence of slag, however, has adverse consequences with respect to both subsequent processing and the final product. For example, liquid (in some cases a solution of a thermally unstable solid) is easily trapped by the slag during cleaning or other wet processing steps. As the liquid trapped by the slag vaporizes during baking, it causes blistering of the substrate. The resulting product is then not usable. Additionally, the slag can cause mechanical weakness by providing an unstable foundation for a subsequently applied layer of conductive metal.